The present invention relates to apparatus and methods of relaxing or loosening textile fabrics. More particularly, the invention relates to apparatus and methods for mechanically relaxing and adjusting the stresses within the fibrous structure of friction-bonded textiles. The apparatus and methods are capable of adjusting or relaxing these stresses by changing the configuration, orientation, or position of certain fibers relative to other fibers. More specifically, the present invention relates to the exertion of lengthwise compressive forces upon traveling lengths of fibrous structures.
Prior commercial devices for compressing fibrous structures or webs in the lengthwise direction have applied driving and retarding forces to the surfaces of textile fabrics by friction and thus have required application of pressure normal to the fabric surface which has impeded fiber movement and relaxation within the structure. Also these devices have been sensitive to variation in thickness and/or density of the fabrics being treated since these factors influence normal pressure and hence the magnitudes of the compressive forces being applied by surface friction. Such prior devices have therefore been largely limited to the processing of light-weight thin fabrics.
In the aforementioned copending application a particular process is disclosed for producing a needle fabric. In a preferred embodiment of that process, fibers are formed into a web of loosely matted fibers. For specific purposes, i.e., producing artificial leather, the web of fibers has a needle pick-up gradient which generally increases in the Z direction, i.e., from the back surface to the face surface of the web. The web is needled into an integral network structure of coherent intensely entangled fibers, wherein the needled structure has an overall bulk density of at least 6 pounds per cubic foot. Preferably, the needled structure has a bulk density which generally increases in the Z direction and the ratio of the bulk density at the face surface to the bulk density at the back surface is at least 2:1 and as high as 8:1, preferably between 3:1 to 5:1. The axis of the flexure of the needled structure, preferably, lies at least within 0.4, e.g., 0.3 and especially 0.2 or 0.1 of the distance from the face surface to the back surface. The needle pick-up gradient, which during needling effects the bulk density gradient, may be accomplished by positioning a plurality of superimposed layers of fibers so that the average fiber denier decreases from the back surface to the face surface and/or is accomplished by positioning the plurality of superimposed layers of fibers so that the average fiber length of the layers decrease from the back surface to the face surface. Onto the face surface of this first so needled fabric may be laid a layer or relatively fine and short, loosely matted fibers and this layer is needled into the previously needled layer until the overall bulk density is at least 8 pounds per cubic foot. The needled fabric may be thereafter wetted with a needling fluid and then further needled while wet to increase the bulk density to at least 10 pounds per cubic foot. The structure is then mechanically relaxed and densified by shrinking the fibers at and adjacent to the face surface. The needled and shrunk fabric is preferably impregnated with a filler, e.g., an elastomer, to an add-on of the dried and/or cured impregnated fabric of between 5% and 200% of the weight of the fabric. The impregnated fabric may be buffed or sanded on the face surface and/or conventional textile or conventional leather finishes are applied thereto, e.g., on the face surface. The face surface may be embossed or otherwise treated as desired. The process may be used to produce an artificial leather.
In connection with the process of the said copending application, it is important that fibers of the fabric structure be loosened and relaxed after the needling has been completed, i.e., some fibers be realigned or slipped relative to other fibers of the structure. Otherwise the tensions on the fibers resulting from the prior processing steps will prevent the needled fabric from shrinking in the desired manner and the directional modulus and forming characteristics and softness or hand of the fabric will not be as desired, e.g., for making artificial leather.
In one aspect, the pulling of the fabric through the needle looms set up a machine direction stress. In another aspect the fiber entanglement process itself orients fibers lengthwise. Accordingly, there is built up a machine direction tensile modulus which is considerably higher than the transverse or widthwise direction modulus. The machine direction and transverse tensile modulus are, therefore, out of balance.
While the relaxation and the balance of modulus in the machine and transverse directions are particularly important for producing the artificial leather of said copending application, the apparatus and methods of accomplishing that relaxation have wider applicability than only artificial leather. Thus, the invention is applicable to any textile structures which are capable of having its fibers moved by internally applied forces, e.g., nonwoven, knitted, and felted fabrics.
Efforts have been made in the art to provide mechanically relaxation of fabrics, especially woven fabrics, and U.S. Pat. No. 2,765,513 to Walton is an earlier example thereof. This approach has been subsequently improved and U.S. Pat. No. 3,426,405 to Walton is an example thereof. In this technique, planar compressive forces are applied to the fabric by action of a frictional moving surface on the surface of the fabric and a slipping engagement of the fabric with a resisting and/or driving surface. Thus, the compressive forces are applied to the surface of the fabric and fibers within the fabric are not directly engaged by the apparatus.
This technique is useful for relatively thin, woven fabrics, where surface forces can be transmitted with some efficiency to the internal fibrous structure, but the technique quickly looses its efficiency with thickness fabrics or with non-woven fabrics, especially fully needled fabrics, wherein the internal fibrous structure is not systematically and relatively uniformly associated with the surface fibrous structure.
As can be appreciated, the ability to mechanically relax or adjust inter-fiber tensions and alignments and give relief from tensile strain within a textile structure would be of considerable benefit to the art since this provides a means of adjusting modulus and improving other properties, such as flexibility, drape, formability, filtering capacity, compactness, resilience, bulk and texture, without the necessity of using solvents, heat, steam, and the like, as often practiced by prior methods.